The present invention relates to fiber-optic switching devices and, more particularly, to a fiber-optic (FO) switch using polarization switching for routing multiple light signals in optical networks such as wavelength division multiplexed (WDM) optical communications, distributed sensor networks, and photonic signal processing systems requiring optical routing and gain control.
The FO switch is a basic building block for many optical applications such as routing in fiber communications networks, photonic signal processing, distributed optical sensing, and optical controls. The desired features for a FO switch include low optical loss (e.g.,  less than 1 dB), low interchannel crosstalk ( less than xe2x88x9230 dB), and simple to align low cost designs for large scale commercial production and deployment. Depending on the application, FO switching speeds can range from nanoseconds to several milliseconds. An attractive technology for making FO switches is via the use of liquid crystal (LC) technology for making light polarization rotators that when placed between polarization sensitive optics forms a switch.
Prior art LC FO switches have suffered from various limitations such as excessive crosstalk, difficult alignment, unbalanced loss between the switch states, and unequal time delays between the switched states. Unequal time delays can cause timing problems when cascading small (e.g., 2xc3x972) switches to make larger Nxc3x97N switches (e.g., N=50) in very wide bandwidth (e.g., 10 Gigabits/sec) fiber-optic communication systems. Crosstalk significantly depends on the quality of the polarization switches. For instance, a major problem with LC based FO switches is that as the LC cell quality goes down, the FO switch crosstalk gets worse. This problem is recognized in the industry and several patents directed to reducing crosstalk have been issued. However, it is not believed that prior patents have adequately resolved the problems arising in this area from fiber alignment. See, for example, A. P. Baker, xe2x80x9cLiquid Crystal Optical Switching Device Having Reduced Crosstalk,xe2x80x9d U.S. Pat. No. 4,720,171, Jan. 19, 1988; J. S. Patel and Y. Silberberg, xe2x80x9cFrequency Selective Optical Switch Employing A Frequency Dispersive Element, Polarization Dispersive Element, and Polarization Modulating Elements,xe2x80x9d U.S. Pat. No. 5,414,540, May 9, 1995; N. K. Shankar, J. A. Morris, C. R. Pollack, C. P. Yakymyshyn, and W. Whitehead, xe2x80x9cOptical Switches Using Cholesteric or Chiral Nematic Liquid Crystals and Method of Using The Same,xe2x80x9d U.S. Pat. No. 4,991,924, Feb. 12, 1991; M. Liebowitz, xe2x80x9cLiquid Crystal Optical Switching Device,xe2x80x9d U.S. Pat. No. 4,917,452, Apr. 17, 1990: R. A. Soref, xe2x80x9cOptical Switches Using Ferroelectric Liquid Crystals ,xe2x80x9d U.S. Pat. No. 4,948,229, Aug. 14, 1990; M. R. Meadows, xe2x80x9cElectro-optical Switch With 4-port Modules With Electro-optic Polarization Rotators,xe2x80x9d U.S. Pat. No. 5,381,250, Jan. 10, 1995; K. T. Yamamoto, Y. H. Itoh, T. Nagayami, xe2x80x9cOptical Space Switch And Network For Such Optical Space Switches,xe2x80x9d U.S. Pat. No. 5,162,944, Nov. 10, 1992; Y. Hakamata, T. Yoshizawa, and T. Kodaira, xe2x80x9cA 1.3xcexcm Single-Mode 2xc3x972 Liquid Crystal Optical Switch,xe2x80x9d IEICE Trans. Commun., Vol. E77-B, No. 10, 1994; R. A. Soref, xe2x80x9cOptical Bypass Switch,xe2x80x9d U.S. Pat. No. 4,478,494, Oct. 23, 1984. xe2x80x9cR. E. Wagner and J. Cheng, xe2x80x9cElectrically Controlled Optical Switch For Multimode Fiber Applications,xe2x80x9d Applied Optics, Vol. 19, pp. 2921-2925, 1980.
Proper use of electronically active and passive polarization components appropriately placed in an optical structure such as a polarization switched optical delay line can greatly suppress structure noise, as suggested in N. A. Riza, xe2x80x9cLiquid crystal-based optical controllers for phased array antennas,xe2x80x9d Optoelectronic Signal Processing for Phased Array Antennas IV Conference Proceedings of the SPIE, Vol. 2155, pp. 169-180, Jan. 1994. Using this concept of proper positioning of polarization components, the patent by K. Y. Wu, xe2x80x9cFault tolerant optical routing switch,xe2x80x9d U.S. Pat. No. 5,724,165, 1998, introduces a low crosstalk 2xc3x972 FO switch. Nevertheless, this transmissive mode switch has high fiber alignment complexity along with an unequal time delay problem between the switch states. Specifically, the two input fibers of the switch have to be simultaneously aligned to the two output fibers through a complex switching structure, making a long duration and tedious alignment task during switch fabrication. Furthermore, it uses several large birefringent beam displacing optics that are costly, adding to the overall expense on building such a switch.
One approach that improves switch alignment is to use an optical structure interconnected with FO circulators. A commercial FO circulator such as available from JDS Fitel, Canada, has three fiber ports. One port is reversible with light flow in and out of the port. The other two ports are a unidirectional input port and a unidirectional output port. Recently, the circulator has been used in a number of liquid crystal-based optical switches where the LC structure for light beam flow control is interconnected with circulators. A Fabry-Perot LC filter is used in the tuning structure proposed in Kuang-Yi Wu, xe2x80x9cTunable Add-Drop Optical Filter,xe2x80x9d U.S. Pat. No. 5,606,439, Feb. 25, 1997. Another similar design but using stacked waveplates is by Kuang-Yi Wu and Jian-Yu Liu, xe2x80x9cSwitchable Wavelength Router,xe2x80x9d U.S. Pat. No. 5,694,233, Dec. 2, 1997.
The present invention addresses the crosstalk and alignment problems of a 2xc3x972 optical switch. The present inventor disclosed an improved fiber-optic switch in a presentation entitled xe2x80x9cFerroelectric liquid crystal polarization switching-based high speed multiwavelength add/drop filters using fiber and array-waveguide gratings,xe2x80x9d OC""98, Optics in Computing European Optical Society Topical Meeting Digest Series, pp. 335-338, Belgium, Jun. 17-20, 1998. The accompanying paper on LC switching describes structure using cube polarization beam splitters (PBSs) for a two circulator structure. It is believed that this is the first time that an LC polarization rotation-based switching structure using a low cost cube PBS was sandwiched between two circulators to form a 2xc3x972 FO switch. Using this retroreflective two circulator approach, the switching structure has only two ports for alignment and not four like in the transmissive 2xc3x972 switches mentioned earlier. Using two circulators makes the switch alignment a simpler task, thus making a more economical device. In addition, the use of low cost cube PBS""s reduces overall switch cost. A further disclosure was made on Jun. 25, 1998 by the present inventor and a co-author in an article entitled xe2x80x9cLow Optical Interchannel Crosstalk, Fast Switching Speed, Polarization Independent 2xc3x972 Fiber Optic Switch using Ferroelectric Liquid Crystals,xe2x80x9d Electronics Letters, Vol.34, No.13, Jun. 25, 1998 by N. A. Riza and Shifu Yuan.
Further improvement on the applicant""s FO switch invention include by N. A. Riza described earlier in N. A. Riza, xe2x80x9cFerroelectric liquid crystal polarization switching-based high speed multiwavelength add/drop filters using fiber and array-waveguide gratings,xe2x80x9d OC""98, Optics in Computing European Optical Society Topical Meeting Digest Series, pp. 335-338, Belgium, Jun. 17-20, 1998, and N. A. Riza and Shifu Yuan. A highly useful module for WDM add-drop routing applications using a unique LC switch structure based on low cost cube PBS""s for sandwiching between two circulators to form 2xc3x972 FO switch invention with low complexity alignment and zero-relative time delay between the switch settings. Additionally, a compact transmissive 2xc3x972 FO switch structure is disclosed for general purpose 2xc3x972 switch applications where the routing channel numbers are small (e.g., 5).
More particularly, the present invention includes a novel 2xc3x972 FO switch structure based on a retro-reflection scheme coupled with FO-circulators, cube PBS""s, and polarization switch devices such as 90 degree twisted nematic liquid crystals (TNLC""s) device incorporating unique passive and active noise filtering techniques inherent in the switch design to make it appropriate for use even when the 90 degree liquid crystal polarization rotation performance is low (e.g., 20 dB extinction ratio). The switch also has the advantage of zero relative time delay between the two channels for any of the two switch settings making it appropriate for cascading switching stages. The use of cube PBS""s reduces cost, while maintaining high crosstalk performance. The inventive 2xc3x972 optical switch is simple in design and assembly in terms of alignment since only two fibers need to be aligned instead of four. It is also scalable to form N 2xc3x972 optical switch modules for use in add/drop WDM networks where its retroreflective characteristic allows the use of half of the required multiplexers/demultiplexers in the system. For smaller (e.g., 10) fiber channel count switching systems, an alternate transmissive 2xc3x972 FO switch is proposed with good crosstalk performance, although with higher optical alignment sensitivity.